The corneal is a layer of transparent film on the front of the eyeball. It often exposed in the air, so it would get more chance of virus infection than other issues of eyes.
The keratitis is a severe eyes disease, it is the important reason of lose some one's sight. The cause of the disease has some reason, trauma and infectious disease of the body will lead to infection of corneal. The virus and anaphylaxis may reduce this disease. The signs of the disease are foreign body sensation, stabbing pain or burning senseation.
As reasons for the corneal ulcer, when the trauma or corneal hurt as well as trachoma complication symptom happens, the virus and bacterials get into the inner of corneal, the corneal ulcer will be reduced. As for the signs of corneal ulcer, the rete vasculosum will get expanded and congestion on the edge of corneal, it named ciliary congestion. The infiltration of corneal will happen at the same time by the invasion of white blood cell. When the infiltration get heavy, the issues of corneal get degeneration, necrosis, or drop, it will become corneal ulcer.
The infiltration and congestion is the same sign of the two eyes disease, but the keratitis will get congestion and the patient may feel photophobia, tears, visual impairment and increasing of incretion; the corneal ulcer will cause corneal get degeneration, necrosis, or drop, it will become corneal ulcer. They are different!
Well, actually, your question is to some extent hard to be answered. As far as I know, those two are both serious diseases which would lead to serious consequences. To be honest, they have lots in common and of course some differences. The former is much more serious and looks more terrible while the latter looks better and not that horrible. My advice for you is to visit a professional eye hospital and get some accurate eye exam.